Method and system of transmission power control

ABSTRACT

The present invention relates to cellular mobile radio systems, and more especially it relates to Code Division Multiple Access, CDMA, cellular mobile radio systems, particularly to transmission power control in such systems. A method and apparatus for transmission of TPC commands when a user equipment is beyond or close to a certain cell radius is disclosed.

This application is the U.S. National phase of international applicationPCT/SEO1/02818 filed 18 Dec. 2001 which designates the U.S.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to cellular mobile radio systems, and moreespecially it relates to Code Division Multiple Access, CDMA, cellularmobile radio systems, particularly to transmission power control in suchsystems.

BACKGROUND AND DESCRIPTION OF RELATED ART

Transmission power control, TPC, transmitting single or multiple TPCbits from a radio base station, RBS, to a mobile station, MS, or viceversa informing the receiving party to increase or decrease thetransmission power level, optionally by a specified amount, ispreviously known.

Transmission power control compensates for signal fading andinterference dynamics at a receiver. Closed loop power control accordingto prior art is described in relation to FIG. 1. In closed loop powercontrol received pilot channel signal to interference ratio, SIR, ismeasured at the receiving end (RBS for uplink). The level of the SIRequals the SIR or a quantized value thereof. The level of the SIR iscompared with a target level. Information on the outcome of thecomparison is fed back in the reverse direction in the form of TPCcommands. Radio wave propagation and power control processing introducesa delay in the feedback loop. To achieve a feedback loop with minimum(one slot) delay, transmission power response and measurement should becontrolled within one slot. The transmitting end adjusts transmissionpower in response to received TPC commands. The receiving end receives,at a propagation delayed time, a pilot signal transmitted at theadjusted level, closing the power control loop. If the level of themeasured SIR is larger than the target level, the receiving endtransmits a command towards the transmitting end (MS for uplink) todecrease the power at the transmitting end. If the level of the measuredSIR is smaller than the target level the transmitting end iscorrespondingly instructed to increase the transmission power. Finally,if the level of the measured SIR is equal to the target level thereceiving end receives no command, a command of no change oftransmission power or interchanging commands of transmission powerincrease and decrease to keep the transmission power close to constant.The command to increase or decrease transmission power is sent by meansof one or more TPC bits. A TPC command determined accordingly and withina predetermined one slot loop delay, irrespective of whether it refersto uplink or downlink power control, is called a regular TPC command inthis patent application.

In FIG. 1, an optional, dependent on slot format and link direction,TFCI field represents a Transport Format Combination Indicator for usee.g. when several simultaneous services are included.

3^(rd) Generation Partnership Project (3GPP): Technical SpecificationGroup Radio Access Network, Physical Layer Procedures, 3G TS 25.214v3.3.0, France, June 2000, specifies in annex B.1 that timing of anuplink dedicated physical channel, DPCH, is delayed by 1024 chips fromthe corresponding downlink DPCH to maximize cell radius within whichone-slot control delay can be achieved. In the sequel this maximumradius is referred to as the one-slot distance to the base station.Basically, a TPC command comprises one bit, indicating a power increaseor decrease. During soft handover there is one such basic TPC bit or TPCcommand for each of the links involved, to be combined into a TPCcommand. Consequently, the concept “TPC command” comprises both suchbasic and combined TPC commands. The 3GPP Technical Specification alsodescribes out of synchronization handling. Briefly, poor quality linksets are indicated to be out of sync. Regarding uplink power control,the MS shall shut its transmitter off during downlink out-of-syncconditions. If the receive timing for any link, during soft handover,drifts to lie outside a valid range, information shall be provided, sothat the network can adjust downlink timing. Regarding downlink powercontrol, during out-of-sync periods the TPC command transmitted shall beset as “1”, i.e. it shall indicate a power increase.

European Patent Application EP0955735 discloses a method, and base andmobile stations for locating transmission power control data and pilotdata in relation to each other within a slot taking into accountprocessing delays and propagation delays and slot offsetting betweenuplink and downlink.

None of the cited documents above discloses a closed loop transmissionpower control, TPC, with the location of TPC data within a slot fixedfor two or more slots in relation to pilot symbols, the power controlbeing adaptive to loop delays varying to be larger than or smaller thanthe duration of one slot.

BRIEF SUMMARY

If the distance between the MS and the RBS is larger than the one-slotdistance, the power control loop delay is larger than the duration ofone slot. If the distance is close to the one-slot distance, it islikely for the control loop delay to vary between one and two slots dueto movements of the mobile station. If the loop delay is larger than oneslot and a TPC command needs to be transmitted within one slot, the TPCcommand will be transmitted prior to estimation of channel quality hasbeen completed.

Correspondingly, if loop delay is larger than two, three, four, etc.slots and a TPC command needs to be transmitted within two, three, fouretc. slots respectively, the TPC command will be transmitted prior toestimation of channel quality has been completed. Consequently, there isa need to assign a TPC command and find a basis for the assignment.

There is a problem in assigning the TPC command such that interferencelevel is not increased and the connection is not lost. If transmissionpower would have been commanded to decrease were the loop delay notlarger than one slot, the interference level to other users couldincrease if the assigned TPC command indicates a power increase.Similarly, if transmission power would have been commanded to increasewere the loop delay not larger than one slot, the connection could belost if the assigned TPC command indicates a power decrease. Therestriction of TPC commands to indicate only a transmission powerincrease or decrease, in accordance with the 3GPP TechnicalSpecification, makes the assignment critical.

The TPC command assignment should take into account a transitional phasewhen the loop delay increases or decreases to pass a slot-border. Itshould also apply in a (quasi-) stationary environment with loop delayslarger than one (or more) slot intervals.

It is consequently an object of the present technology to achievetransmission power control that is stable when transmission powercontrol loop delay increases beyond a slot border.

It is also an object to achieve a system stabilized for loop delaysvarying so as to be larger than the duration of one slot interval in oneinstance and not larger than one slot interval in the next instance andvice versa.

An object is also to have a transmission power control operating at aloop delay as small as possible.

A further object of the present technology is to remedy a too high ortoo low transmission power level.

Finally, it is an object to achieve a stable transmission power controlusing fixed location relation within a slot for uplink and downlink andusing a fixed timing relation between uplink and downlink.

These objects are met by a method and apparatus transmitting a TPCcommand based upon a latest earlier measurement, not yet made use of, ifavailable, and if not available, transmitting a TPC command identical toa most recently transmitted TPC command at its first occurrence andtransmitting a TPC command corresponding to an inversion of a mostrecently transmitted TPC command at later occurrences.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 displays slot content and timing, according to prior art, oftransmission of TPC commands and subsequent response to TPC.

FIG. 2 illustrates an incrementally commanded power level versus time,with a loop delay not larger than one slot.

FIG. 3 illustrates an incrementally commanded power level versus time,with loop delays varying between not larger than one slot and largerthan one slot but not larger than two slots, according to a preferredembodiment.

FIG. 4 illustrates another sample of commanded power level versus time,in the case of a received quality level varying in relation to a targetlevel with a loop delay not larger than one slot.

FIG. 5 illustrates an incrementally commanded power level versus time,with loop delays identical to those of FIG. 3, according to a preferredembodiment.

FIG. 6 shows a flowchart of a preferred embodiment.

FIG. 7 displays the principle of assigning a TPC command when nomeasurement data of the same slot is available, according to a preferredembodiment.

FIG. 8 schematically illustrates a mobile station and two radio basestations.

DESCRIPTION OF PREFERRED EMBODIMENT

In particularly a CDMA system it is important to control the uplink (anddownlink) transmission power to a level not larger than necessary inorder to keep the interference level and power consumption of the systemat a minimum.

A candidate solution to avoid varying loop delays is to insert anadaptive delay for respective up- and downlink power control loops inthe RBS or MS to operate the system at a (close to) fix loop delay equalto the largest loop delay allowed, preventing the total delay to pass aslot border, i.e. preventing it from increasing or decreasing by theduration of one slot or more. However, in most cases such a solutionleads to an excessive loop delay, detrimental to system performance.

The previous TPC command is repeated if no measurement data, upon whichno previous TPC command is based, is available in a first occurrence. Ifno measurement data is available in later occurrences, the previous TPCcommand is inverted. The repetition, in the first occurrence, willguarantee that the inversion process does not increase transmissionpower to a level higher than the power level at a point in time wherethe delay passed the slot border, if the previous TPC command indicateda transmission power decrease. Correspondingly, the repetition, in thefirst occurrence, will guarantee that the inversion process does notdecrease transmission power below the power level at the time when loopdelay increased beyond a slot border, if the previous TPC commandindicated a transmission power increase. This property is important, forthe inversion process not to be the cause of an increased interferencelevel or a lost connection.

Typically, as received signal quality deteriorates to a lowestacceptable quality level, a transmitter is commanded to increasetransmission power. If the next command is also an increase, there is norisk of loosing the connection caused by the power control commandingpower to decrease. A corresponding but reversed situation occurs whentransmission power is larger than necessary and increasing an overallinterference level to be decreased by power control. In this reversesituation, a penalty of using an assignment not behaving well wouldstrike other users interfered with.

FIGS. 2 and 4 each reveals a resulting commanded power level versus timefor a particular received signal quality varying over time when powercontrol loop delay remains within one slot. The selection of one singleslot is only an example. The same principle holds for successivelylarger distances, i.e. varying the loop delay between two and threeslots, three and four slots, etc. In FIG. 2, transmission power iscommanded to increase until slot S1. According to the received signalquality, transmission power is commanded to decrease in the intervalbetween S1 and S2. At slot S2 the commanded transmission power is againcommanded to increase as a response to a decreased received signalquality. In FIG. 4 increases and decreases are interchanged in relationto FIG. 2.

FIG. 3 describes a resulting commanded power level versus time for areceived signal quality corresponding to that of FIG. 2. For each powerlevel the loop delay is indicated by “1” or “2”. “1” denotes a loopdelay larger than zero but not larger than one slot. “2” denotes a loopdelay larger than the duration of one slot but not larger than theduration of two slots. The loop delay is indicated correspondingly inFIG. 5. In FIGS. 3 and 5 no measurement data not forming a basis of aprevious TPC command is available for the time slots indicated by “2”.

In FIG. 3 the problem of assigning a TPC command with no measurementdata available and simultaneously avoiding the risk of increasing thecommanded power level is solved by repeating the immediately precedingTPC command value when the slot border is passed due to an increasedpower control loop delay. Consequently, if the immediately preceding TPCcommand is a regular TPC command transmitted with a loop delay withinone slot interval as in slot Sa, this TPC command value is repeated, asillustrated in slot Sb. In the proceeding slot interval Sc, theimmediately preceding TPC command in slot Sb is a repetition of aprevious TPC command in slot Sa and not a TPC command assigned on thebasis of <<most recent measurement data not previously forming a basisfor a TPC command>>. According to the preferred embodiment, the TPCcommand in slot Sc is an inversion of the previously repeated TPCcommand in slot Sb. Since the loop delay remains larger than one slotinterval when the next TPC command is scheduled for transmission in slotSd, the transmitted TPC command will be an inversion of its immediatelypreceding bit. Interchanging TPC command inversions are transmitteduntil the loop delay is not larger than one slot interval. Then aregular TPC command is transmitted, as in slot Se.

Referring to FIG. 5, the receiver is enforced to communicate a powercontrol command within the required number of slots (in this exemplarypresentation one slot) by transmitting a TPC command with no measurementdata available. Similar to FIG. 3, the problem is solved by repeatingthe immediately preceding TPC command when the slot border is passed dueto an increased power control loop delay and interchangingly invertingthe TPC command in subsequent slots until the loop delay exceeds oneslot interval.

FIG. 6 shows a flowchart of a preferred embodiment. Upon communicationof a TPC command the receiver investigates whether it can respond withinone slot interval. When the loop delay is determined to be within theslot interval, the receiver communicates a regular TPC command. The loopdelay can be estimated by the fact that expected measurement data is notavailable at the time of assignment of the TPC command. If the slotborder is exceeded, the TPC command to communicate depends on thepreviously transmitted TPC command. If the previous TPC command was aTPC command determined from <<most recent measurement data notpreviously forming a basis for a TPC command>>the receiver communicatesthis TPC command. If the previous TPC command was not such a TPC commandthe receiver communicates an inversion of the previous TPC command asdescribed in relation to FIGS. 3 and 5, above. If the previous TPCcommand indicated a power increase, its inversion will indicate a powerdecrease. Correspondingly, if the TPC command indicated a powerdecrease, its inversion will indicate a power increase.

FIG. 7 displays the principle of assigning a TPC command according tothe flowchart in FIG. 6 in a preferred embodiment. In FIG. 7 a sample ofTPC commands 1-12 for uplink power control is displayed versus time. Thetiming is illustrated, as it is perceived at a base station. Theillustrated commands are transmitted on the downlink. However, there isno fundamental difference between downlink power control and uplinkpower control. Consequently, the following explanation of FIG. 7 alsoapplies to downlink power control, interchanging the roles of RBS andMS. The uplink and downlink timing includes two components, a fixuplink-downlink timing offset and a distance dependent round trippropagation and processing delay. In the first slot illustrated in FIG.7 this uplink-downlink timing equals T.sub.do. In FIG. 7 thisuplink-downlink timing varies due to varying propagation delays invarious time slots. 1-3 are regular TPC commands. TPC command 2 is basedon the uplink quality measured on a pilot sequence b. Correspondingly,TPC command 3 is based on the uplink quality measured on a pilotsequence c. The measurements on pilot sequences b and c are completedprior to transmission of TPC commands 2 and 3, respectively. Inequivalent words, TPC commands 2 and 3 are both within a loop delay notlarger than the duration of one slot. At time t.sub.0 loop delayincreases from not larger than the duration of one slot to larger thanthe duration of one slot. When TPC command 4 needs to be transmittedthere is no measurement available from the corresponding pilot sequenced since the loop delay is larger than the duration of one slot. Further,the measurement from pilot sequence c, the most recent earlier slot, hasalready been used as a basis for forming TPC command 3. As the previousTPC command 3 was based on measurement data, TPC command 4 is assigned avalue identical to the previous TPC command 3, i.e. TPC command 3 isrepeated. As TPC command 5 needs to be transmitted, there is again nomeasurement data available within a loop delay of one slot, i.e. fromthe corresponding pilot sequence e. However, there is measurement dataon pilot symbols from the previous slot d, this measurement data has notbeen made use of and there is no later available measurement data thathas not been made use of. This measurement data forms the basis for TPCcommand 5. At time t.sub.1 loop delay reduces and is not larger than theduration of one slot when TPC command 6 needs to be determined.Measurement data from the pilot sequence f of the corresponding slot isavailable at the time TPC command 6 is assigned. In the sequelmeasurement data M from pilot sequence e will never again be a mostrecent earlier measurement data not used as a basis for assignment of aTPC command, within the meaning of this patent application, asmeasurement data on f is more recent. Consequently, when TPC command 7needs to be assigned there is no measurement data on the pilot sequenceg of the corresponding slot available, and there is no <<most recentmeasurement data not previously forming a basis for a TPC command>>. Asthe previous TPC command 6 was based on measurement data (from f), TPCcommand 7 is assigned a value identical to TPC command 6. TPC command 8is formed on basis of the measurement data from pilot sequence g notpreviously made use of. As TPC commands 9, 10 and 11 needs to beassigned there is no <<most recent measurement data not previouslyforming a basis for a TPC command>> available for the assignment. TPCcommand 8 is not achieved from unused measurement data but fromrepeating TPC command 7. Consequently, TPC commands 9, 10 and 11 areassigned the inversion of TPC commands 8, 9 and 10 respectively. Thisinvention is not limited to any particular reason, for which nomeasurement data is available during intervals h, i and j. One reasoncould be that MS has shut off its transmission power, another reasoncould be that uplink is out-of-sync due to poor receiver or linkconditions (cf. FIG. 8).

FIG. 8 schematically illustrates a subsystem including an MS and twoRBSs. The system instruments the embodiments described above. In thisfigure the RBSs are indicated to operate omnidirectionally. However, theinvention is not limited to omnidirectional radio base stations. It canreadily be used irrespective of whether the RBSs use directional oromnidirectional antenna radiation patterns. A first radio base stationRBS 1 is provided, according to the preferred embodiment, with means 1for detection of link quality and means 2 for transmission of a first,second or third power control command. RBS 1 and RBS 2 are equipped withlogic as needed to decide on which power control command to transmit.Means 1 and 2 can be included in the RBS or connected as one or moreseparate devices. The MS is furnished with means 3 for identifying andresponding to received power control commands. The radio base stationRBS 1 or RBS 2 receives signals on an uplink. The received signalquality and loop delay forming a basis of the TPC command. The TPCcommand is transmitted on the downlink for transmission power control ofthe MS, so called uplink power control. If the invention is also appliedfor downlink power control, transmission power of the RBS is controlledcorrespondingly reversing the roles of the RBS and MS. For thissituation the MS is illustrated including means 4 and 5 corresponding tomeans 1 and 2 respectively of the RBS and the RBS is furnished withmeans 6 corresponding to means 3 of the MS. RBS 1 and RBS 2 includestorage means 7 for storage of measurement data. The MS includescorresponding storage means 8.

A person skilled in the art readily understands that the receiver andtransmitter properties of an RBS or an MS are general in nature. The useof concepts such as RBS or MS within this patent application is notintended to limit the invention only to devices associated with theseacronyms. It concerns all devices operating correspondingly, or beingobvious to adapt thereto by a person skilled in the art, in relation tothe invention. As an explicit non-exclusive example the inventionrelates to mobile stations without a subscriber identity module, SIM, aswell as user equipment including one or more SIMs.

The invention is not intended to be limited only to the embodimentsdescribed in detail above. Changes and modifications may be made withoutdeparting from the invention. It covers all modifications within thescope of the following claims.

1. A method of communicating power control commands from a receiver to atransmitter wherein a first power control command is communicated whenthere is most recent measurement data not previously forming a basis fora power control command available at a specified point in time, and asecond or third power control command is communicated when no mostrecent measurement data not previously forming a basis for a powercontrol command is available at the specified point in time, and whereinmeasurement data not yet forming a basis of a power control command isstored until new measurement data is collected or the measurement datais used for forming a basis of a power control command.
 2. A method ofcommunicating power control commands from a receiver to a transmitterwherein a first power control command is communicated when there is mostrecent measurement data not previously forming a basis for a powercontrol command available at a specified point in time, and a second orthird power control command is communicated when no most recentmeasurement data not previously forming a basis for a power controlcommand is available at the specified point in time, and wherein thesecond power control command is communicated when the immediatelypreceding power control command was a first power control command. 3.The method according to claim 2 characterized in that the first powercontrol command is determined on the basis of measurement data receivedmost recently and prior to the specified point in time; the measurementdata received not previously serving as a basis of determining a powercontrol command.
 4. The method according to claim 2 characterized inthat the receiver is a radio base station, or is included in orconnected to a radio base station.
 5. The method according to claim 2characterized in that the receiver is a mobile station, or is includedin or connected to a mobile station.
 6. A radio communication systemcomprising means for carrying out the method of claim
 2. 7. A method ofcommunicating power control commands from a receiver to a transmitterwherein a first power control command is communicated when there is mostrecent measurement data not previously forming a basis for a powercontrol command available at a specified point in time, and a second orthird power control command is communicated when no most recentmeasurement data not previously forming a basis for a power controlcommand is available at the specified point in time, and wherein thethird power control command is communicated when the immediatelypreceding power control command was a second or a third power controlcommand.
 8. A method of communicating power control commands from areceiver to a transmitter wherein a first power control command iscommunicated when there is most recent measurement data not previouslyforming a basis for a power control command available at a specifiedpoint in time, and a second or third power control command iscommunicated when no most recent measurement data not previously forminga basis for a power control command is available at the specified pointin time, and wherein the second power control command is identical tothe immediately preceding power control command.
 9. A method ofcommunicating power control commands from a receiver to a transmitterwherein a first power control command is communicated when there is mostrecent measurement data not previously forming a basis for a powercontrol command available at a specified point in time, and a second orthird power control command is communicated when no most recentmeasurement data not previously forming a basis for a power controlcommand is available at the specified point in time, and wherein thethird power control command is identical to the immediately precedingpower control command inverted.
 10. A method of communicating powercontrol commands from a receiver to a transmitter wherein a first powercontrol command is communicated when there is most recent measurementdata not previously forming a basis for a power control commandavailable at a specified point in time, and a second or third powercontrol command is communicated when no most recent measurement data notpreviously forming a basis for a power control command is available atthe specified point in time, and wherein the specified point in time isequal to or related to the point in time of assignment of a TPC command.11. A method of communicating power control commands from a receiver toa transmitter wherein a first power control command is communicated whenthere is most recent measurement data not previously forming a basis fora power control command available at a specified point in time, and asecond or third power control command is communicated when no mostrecent measurement data not previously forming a basis for a powercontrol command is available at the specified point in time, and whereinthe specified point in time corresponds to a largest loop delay equal tothe duration of an integer multiple of the duration of one slotinterval, the integer multiple being at least one.
 12. A receiverelement including means for communication of power control commands to atransmitter, the receiver element comprising: means for determining whenthere is available most recent measurement data; means for communicationof a first power control command when the most recent measurement datais available and for communication of a second or third power controlcommand when no most recent measurement data is available, the thirdpower control command being identical to an immediately preceding powercontrol command inverted; means for communication of the second powercontrol command when the immediately preceding power control command wasa first power control command.
 13. A receiver element including meansfor communication of power control commands to a transmitter, thereceiver element comprising: means for determining when there isavailable most recent measurement data; means for communication of afirst power control command when the most recent measurement data isavailable and for communication of a second or third power controlcommand when no most recent measurement data is available, the thirdpower control command being identical to an immediately preceding powercontrol command inverted; means for communication of the third powercontrol command when the immediately preceding power control command wasa second or a third power control command.
 14. The receiver elementaccording to claim 13, characterized by means for determining the firstpower control command on the basis of the measurement data received mostrecently.
 15. The receiver element according to claim 13 characterizedin that the receiver is a radio base station, or is included in orconnected to a radio base station.
 16. The receiver element according toclaim 13 characterized in that the receiver is a mobile station, or isincluded in or connected to a mobile station.
 17. A receiver elementincluding means for communication of power control commands to atransmitter, the receiver element comprising: means for determining whenthere is available most recent measurement data; means for communicationof a first power control command when the most recent measurement datais available and for communication of a second or third power controlcommand when no most recent measurement data is available, the thirdpower control command being identical to an immediately preceding powercontrol command inverted; wherein the second power control command isidentical to the immediately preceding power control command.
 18. Areceiver element including means for communication of power controlcommands to a transmitter, the receiver element comprising: means fordetermining when there is available most recent measurement data; meansfor communication of a first power control command when the most recentmeasurement data is available and for communication of a second or thirdpower control command when no most recent measurement data is available,the third power control command being identical to an immediatelypreceding power control command inverted; wherein the third powercontrol command is identical to the immediately preceding power controlcommand inverted.
 19. A receiver comprising: a loop delay detectorconfigured to determine when a loop delay between the receiver and atransmitter is within a transmission slot interval; a power controlcommand generator configured to generate a power control command fortransmission to the transmitter, and wherein the power control commandas generated by the power control command generator is either: (1)dependent upon a measurement made within the transmission slot intervalwhen the loop delay is within the transmission slot interval; (2) arepetition of a most recently transmitted power control command when theloop delay is not within the transmission slot interval and the mostrecently transmitted power control command was based on measurement notpreviously forming a basis for a power control command; (3) dependentupon measurement data which has been not utilized for a power controlcommand when the loop delay is not within the transmission slotinterval; (4) an inversion of the most recently transmitted powercontrol command when the loop delay is not within the transmission slotinterval and the most recent measurement data previously formed a basisfor a power control command.
 20. The apparatus of claim 19, wherein thereceiver comprises a radio base station.
 21. The apparatus of claim 19,wherein the receiver comprises a wireless terminal.
 22. A receiverelement comprising: means for communication of power control commands toa transmitter; means for determining when most recent measurement datais available; means for communication of a first power control commandwhen the most recent measurement data is available and, when no mostrecent measurement data is available, for making a selection between asecond power value or a third power value, the selection depending onhow long measurement data has been unavailable; means for storage ofmeasurement data not yet forming a basis of a power control command. 23.A receiver element comprising: means for communication of power controlcommands to a transmitter; means for determining when most recentmeasurement data is available; means for communication of a first powercontrol command when the most recent measurement data is available and,when no most recent measurement data is available, for making aselection between a second power value or a third power value, theselection depending on how long measurement data has been unavailable;wherein the second power value is a repeat of a previous power value andthe third power value is an inversion of the previous power value.